1. Field of Invention
This invention relates to improvements in H-bridge driver circuits for driving an inductive load, and more particularly to improvements in driver circuits of the type described that have circuits for diverting current from flowing through a high side driver transistor from the inductive load upon switching.
2. Relevant Background
In systems in which an inductive load is being driven by an H-bridge configuration of FETs, there is the potential for the inductive load to source current through the high-side (HSD) FET body diode and onto the main supply line. This can result in increased supply voltage and damage to components not rated for such a voltage spike. The inductor current being directed to the main supply line must be absorbed by either the main supply or the line load.
Unfortunately, many power supplies exhibit poor or no ability to sink the inductor current. The line load generally consists of a resistive load, a current load, and a capacitive load. The current load can do nothing to absorb the current, and the resistive load can only do so at the price of increasing the voltage on the supply line. If enough capacitance is added to the system, then the capacitance can sink the inductor current, but this is a costly solution. Purchasers of drive electronics which are driving inductive loads would benefit if the drive electronics were able to prevent the inductive current from dumping onto the main supply line.
To address this problem in the past, others have used a zener diode from the middle of the H-bridge to the gate of the low side driver. When the inductor current pushes current into the middle of the H-bridge, the voltage increases, and if the voltage increases enough then the Zener diode breaks down and the low side driver is turned on to shunt the inductor current to ground. However, when using this method it is difficult to control the clamping level of the middle of the H-bridge because it is governed by the threshold voltage of the low side driver power device, the current handling ability of the low side driver, and the breakdown voltage of the zener diode. All three of these parameters can vary greatly from part to part. In some cases the magnitude of the variation for each parameter can be above 15%. This results in a poorly controlled clamping voltage. What is needed, therefore, is a circuit and method to protect the power supply and its components from damaging currents that may be produced by an inductive load in an H-bridge environment. This circuit and method should safely divert damaging currents away from the supply and into the ground in a well regulated manner such that the level at which the middle of the H-bridge is clamped is well controlled.
Herein proposed is a solution in which the current from the inductive load is redirected in a regulated fashion to keep the current from pumping up the supply line voltage. The solution conceptually consists of a transconductance loop which regulates the voltage on a low side (LSD) gate to pull current from the inductive load to ground instead of allowing it to flow to the supply. The LSD gate being regulated is on the same half H-bridge as the HSD gate whose body diode is supplying the path for the inductive load to pump up the supply line.
In light of the above, therefore, an advantage of the invention is that by using a transconductance loop approach, the clamping voltage can be made to depend only on a reference voltage, eliminating the dependence of the clamping voltage on transistor characteristics such as threshold voltage and current handling capability. The proposed invention also does not use a Zener diode so there is no dependence on the Zener diode breakdown voltage, which may be poorly controlled.
Another advantage of the invention is that by allowing the clamping voltage to depend only on a reference voltage, the designer can easily design the H-bridge drivers to clamp at any voltage necessary, without being limited to the available zener breakdown voltages and transistor threshold values.
Thus, a circuit is presented that includes circuitry to operate an H-bridge configuration of FETs that are driving an inductive load. A half H-bridge consists of a two transistors: a high side transistor and a low side transistor. The high side transistor has the supply voltage as its drain, the midpoint of the half H-bridge as its source, and an on-off control signal connected to its gate. The low side transistor has the midpoint of the half H-bridge as its drain, the ground (or other sufficiently low voltage) as its source, and the invention herein-described connected to its gate. The inductor has the midpoint of the half H-bridge connected to one of its terminals, and the second terminal is free to connect to any node as long as this second node can carry current.
According to a broad aspect of the invention, a circuit is presented for use in an electrical circuit that includes an inductive load which has a first side driven by an FET pair including a high side transistor connected in series with a low side transistor between a supply voltage and a reference potential. A second side of the inductive load is connected to a circuit capable of carrying current to a reference potential. The high side transistor is of the type that has an associated body diode. The circuit includes circuitry to operate the high side transistor and the circuit capable of carrying current to selectively allow a current to flow from the supply voltage through the high side transistor, the inductor, and the circuit capable of carrying current. A transconductance circuit loop is connected to control the circuit capable of carrying current to pull current from the inductive load to the reference potential when the inductive load sources current to the body diode sufficient to cause the voltage across the body diode to exceed a predetermined trip voltage.
A transconductance loop regulates the voltage on the gate of one of the low side transistors to cause that same low side transistor to regulatedly conduct the inductor current away from the high side transistor to ground. It may include a sense circuit connected to sense the common voltage between a high side driver and a low side driver connected in series to provide a sensed voltage and a compare circuit connected to compare a trip voltage with the sensed voltage to provide an output current to a control element of the low side transistor to cause the low side transistor to conduct an amount of current related to a magnitude of the sensed voltage over the trip voltage.
According to another broad aspect of the invention, a method is presented for operating an H-bridge power circuit that provides power to a load that includes an inductive component. The method includes sensing a voltage between a high side driver and a low side driver of the H-bridge power circuit and, if the voltage exceeds a predetermined level, diverting a current from the high side driver to the low side driver. The diversion of current may be accomplished by diverting the current from the high side driver to the low side driver in relation to a magnitude by which the voltage exceeds the predetermined level.
According to still another broad aspect of the invention, a circuit is provided for protecting the upper transistors of an H-bridge circuit that provides power to a load that has an inductive component. The circuit includes a transconductance circuit loop connected between one side of the inductive component and the gate of a low side transistor of the H-bridge. The transconductance circuit operates to pull current from the inductive component to ground when the inductive load sources current to a body diode of the high side transistor. The transconductance loop creates a regulated voltage to the gate of the low side transistor to cause the low side transistor to regulatedly conduct the current away from the inductor and the high side transistor to ground.
According to still yet another broad aspect of the invention, an H-bridge power circuit is presented for providing power to a load that includes an inductive component. The H-bridge power circuit includes means for sensing a voltage between a high side driver and a low side driver of the H-bridge power circuit, and means for diverting a current from the high side driver to the low side driver if the voltage exceeds a predetermined level. The means for diverting may include means for diverting the current from the high side driver to the low side driver in relation to a magnitude by which the voltage exceeds the predetermined level. The means for diverting may also include means for applying a bias to a control element of the low side driver in relation to a magnitude by which the voltage exceeds the predetermined level. The means for diverting may also include means for developing a regulated bias current and applying the bias current to a control element of the low side driver.